Anomalous Propagation
Greater density slows the waves more.
Less dense air does not slow the waves as
much.
Since density normally decreases with
height, the radar beam is refracted toward
the surface of the Earth.
Refraction
Subrefraction
If the decrease in density with height is more
than normal, then the beam bends less
than normal and this is called
subrefraction.
In this case the beam might shoot over the
target and miss the precipitation.
Subrefraction (Cont.)
Superrefraction
If the decrease in density with height is less
than normal, then the beam bends more
than normal and this is called
superrefraction.
In this case the beam bends more toward
the surface of the Earth, and it may
undershoot the target.
Superrefraction (Cont.)
Ducting
If the decrease in density with height is
much less than normal, the beam may
bend down to the surface of the Earth in a
process called ducting.
If the beam is backscattered to the receiver,
it may result in Anomalous Propagation
(AP) or “false echoes”.
Ducting (Cont.)
How to read the intensity scale
Extreme
Intense
Severe
♦ Units are decibels of Z
(reflectivity).
Heavy
, Insects, Birds
Light Precipitation
Very light precipitation
Moderate
Light
Fog, Clouds, Smoke
Dust (?? Skeptic)
Very light
Precipitation
Mode Scale
Clear-Air Scale
Hail Detection
• Returns > 55 dBz usually indicate hail.
• However, the probability of hail reaching
the ground depends on the freezing altitude.
• Usually, a freezing level above 4300 m
(14,000 feet) will not support much hail.
• This is because the hail melts before
reaching the ground.
• Freezing level can be determined from an
upper air sounding.
Hail?
Max return of 60 dbZ
Max return of 65 dbZ
Freezing level was 2100 m
Freezing level was 5200 m
(7,000 feet)
(17,000 feet)
Produced golfball sized hail
Produced no hail
Hence, hail production depends directly on freezing level.
Vertically Integrated Liquid (VIL)
■ Take a vertical column of the atmosphere: estimate
the amount of liquid water in it.
■ High VIL values are a good indication of hail.
• The white pixel indicates a VIL
of 70.
• This storm produced golfball
size hail.
• Drawback: complete scan
required (time)
The Hail Spike
Also called Three-Body Scattering
▪ A dense core of wet hail will reflect part of the beam to the ground, which
then scatters back into the cloud, and is bounced back to the antenna.
▪ The delayed returns trick the radar into displaying a spike past the core.
▪ Usually, will only result from hail 1 inch in diameter or larger (quarter size).
Echo Tops
Fairly accurate at depicting height of storm tops
Inaccurate data close to radar because there is no
beam angle high enough to see tops.
Often has stair-stepped appearance due to uneven
sampling of data between elevation scans.
Precipitation Estimates
Storm Total
Precipitation
● Total estimated
accumulation for a set
amount of time.
● Totals are in inches
● Time range is sometimes
listed on image.
● Resets storm total
whenever there is no rain
detected for an hour.
One Hour Precipitation Total
-Updated once per
volume scan.
-Shows accumulated
rainfall for the last
hour.
-Useful for
determining rainfall
rate of ongoing
convection.
Precipitation Estimate
Advantages
● Great for scattered areas of
rain where no rain gauges are
located
● Has helped issue flash flood
warnings more efficiently
● Helps fill in the holes where
ground truth information is not
available
● Much better lead time for
warnings
● Provides a graphical ‘map’ of
rainfall for an entire region
● Data can be overlaid with
terrain and watersheds to
predict reservoir and waterway
crests
and
Limitations
● Estimates based on cloud water
levels and not ground level rainfall
● ‘Hail Contamination’ causes
highly inflated values
● High terrain causes
underestimates
● Lower resolution than
reflectivity images
● Useful as a supplement, not
replacement for ground truth
information
How Doppler Wind Is Displayed
Inbound velocities (towards the radar) are shaded blue, with pale shades for light
winds and dark shades for strong winds. Outbound velocities (away from the
radar) are shaded orange with pale shades (yellow) for light winds and dark
shades (red) for strong winds.
Warm colors are
winds moving
away from
radome
(reds, +)
Velocity Imagery
Cool colors are
winds moving
toward radome
(greens, -)
Wind
speed is
in knots
Tight area of opposing winds (+ and -) can indicate convergence
or rotation. Circled area called a couplet. Indicates a possible
tornado.
Detecting Rotation
A velocity couplet may indicate rotation.
+ (radar site)
outound radial
velocities
inbound radial
velocities
X
Detection Rotation (Cont.)
Not all velocity couplets indicate rotation.
inbound radial
velocities
+ (radar site)
outbound radial
velocities
Linear (straight line) flow over the radar site.
Bragg Scatter
s 
r
2sin 
Bragg Scatter
Clear-Air Turbulence
Clear-Air Wind Profilers
Positive Toward
Surface
Radial Velocity
Signal Power
NWS-NOAA Profiler Network
SODAR
SODAR
Parameter
Performance
Horizontal wind speed components
Range 0-20 m/s, accuracy 0.2 m/s
Horizontal wind speed vectors
Range 0 - 25 m/s
Vertical wind speed components
Range 0 - 10 m/sec, accuracy 0.1 m/s
Horizontal wind Direction
0 - 359 degrees
Resolution of reading
0.1 m/s
Sampling Height
50 m - 900 m AGL
Environmental Conditions
-10C to + 40C, 0 - 100% humidity
Acoustic Frequency
1525 - 2225 Hz (selectable)
Radio-Acoustic Sounding
(RASS)
Energy Absorbed by Atmosphere
94 GHz
35 GHz
Maximum
Propagation
Distance
10-15 km
20-30 km
3.2 mm
8 mm
Radar Wavelength
The DOE Cloud Radars
Cloud Radar Data from Southern Great Plains
20-km
Black Dots:
Laser
Measurements
Of Cloud
Base Height
10-km
Surface
7:00 pm
Small Cloud Particles
time
7:00 am
Typical Cloud Particles
7:00 pm
Very Light Precipitation
Cloud Radar Data from Southern Great Plains
20-km
Black Dots:
Laser
Measurements
Of Cloud
Base Height
10-km
Thin
Clouds
Insects
Surface
7:00 pm
Small Cloud Particles
time
7:00 am
Typical Cloud Particles
7:00 pm
Very Light Precipitation